A conventional split Stirling refrigeration system is shown in FIG. 1. This system includes a reciprocating compressor 5 and a cold finger 6. The piston 7 of the compressor provides a near sinusoidal pressure variation in a pressurized refrigeration gas such as helium. This pressure variation is transmitted from the head space 8 to the cold finger 6 through a supply line 9.
The usual split Stirling system includes an electric motor driven compressor. A modification of that system is the split Vuilleumier system. In that system, a thermal compressor is used. This invention is applicable to both of those refrigerators as well as others such as Gifford-MacMahon refrigerators.
The cold finger 6 houses a cylindrical displacer 10. This displacer is free to move in a reciprocating motion. This motion is responsive to changes of volumes in a warm space 11 and a cold space 12. Contained within the displacer is a regenerative heat exchanger 13 comprised of several hundred fine mesh metal screen discs stacked to form a cylindrical matrix. Heat exchangers are also known to be comprised of stacked balls. This construction allows helium to flow through the regenerator between the warm space 11 and the cold space 12.
With the movement of the compressor piston 7, the pressure in the working volume 8, 9, 11, 12, 13 begins to rise from a minimum pressure to a maximum pressure. As a result, the temperature of the working volume of gas begins to rise. Heat created by the increase in pressure is then transferred to the environment; as a result, the compression at the warm end 11 of the cold finger is nearly isothermal. As the pressure increases, a pressure differential across the displacer in the cold finger is created. When retarding forces are overcome, the displacer is free to move within the cold finger. Along with the movement of the displacer, this pressure differential forces the working gas from the warm end of the cold finger through the regenerator and into the cold space 12. As the pressurized refrigerant gas flows through the regenerator, heat is absorbed. Thus, the temperature of the gas is cooled.
When the compressor piston reverses its direction and begins to expand the volume of gas in the working volume, the helium in the cold space is cooled even further. It is this cooling at the cold end of the displacer which provides refrigeration for maintaining a time average temperature gradient of over 200 degrees Kelvin over the length of the regenerator.
As the gas in the working volume is expanded, a pressure differential is created from the cold space to the warm space. When retarding forces are overcome, the displacer is free to move towards the warm space. This pressure differential also forces the working gas from the cold space through the regenerator and into the warm space 11. Heat is absorbed from the regenerator by the gas as it passes through to the warm end of the cold finger. As a result, the displacer is returned to its starting position.
More recently, refrigerators have been proposed and manufactured that depend on linear motor systems to control the movement of the piston or pistons in the compressor and that of the displacer. For example, in the patent application of Niels O. Young, Ser. No. 458,718, now U.S. Pat. No. 4,545,209, each compressor includes a permanent magnet mounted to the moving armature of the motor which, in turn, drives a piston element.
A goal in the use of these linear motor refrigerators is to produce a more efficient refrigerator which is both easier and less expensive to manufacture.